1. Field of the Invention
The present invention relates to a superconducting active lumped component for microwave device application, and particularly to a superconducting active lumped component for microwave device application of which properties can be changed during operation.
2. Description of Related Art
Electromagnetic waves called "microwaves" or "millimetric waves" having a wavelength ranging from a few tens of centimeters to a few millimeters can be theoretically said to be merely a part of an electromagnetic wave spectrum, but in many cases, have been considered from the viewpoint of electrical engineering as being a special independent field of the electromagnetic waves, since special and unique methods and devices have been developed for handling these electromagnetic waves.
In the case of propagating an electromagnetic wave in microwave and millimetric wave frequency bands, a twin-lead type feeder used in a relative low frequency band has an extremely large transmission loss. In addition, if an inter-conductor distance approaches a wavelength, a slight bend of the transmission line and a slight mismatch in connection portion cause reflection and radiation, and is easily influenced from adjacent objects due to electomagnetic interference. Thus, a tubular waveguide having a sectional size comparable to the wavelength has been conventionally used. The waveguide and a circuit constituted of the waveguide constitute a three-dimensional circuit, which is larger than components used in ordinary electric and electronic circuits. Therefore, use of microwave circuits has been limited to special fields.
However, miniaturized devices composed of semiconductor materials have been developed as an active element operating in a microwave band. In addition, with the advancement of integrated circuit technology, so-called microstrip lines having a extremely small inter-conductor distance have been used.
In general, the microstrip line has an attenuation coefficient that is attributable to a resistance component of the conductor. This attenuation coefficient, attributable to the resistance component, increases in proportion to a root of the frequency. On the other hand, the dielectric loss increases in proportion to increase of the frequency. However, the loss in more recent microstrip lines is attributable almost exclusively to the resistance of the conductor in a frequency region not greater than 10 GHz, due to the improvement dielectric materials. Therefore, if the resistance of the conductor in the strip line can be reduced, it is possible to greatly elevate the performance of the microstrip line. Namely, by using a superconducting microstrip line, the loss can be significantly decreased and microwaves of higher frequency range can be transmitted.
As well known, the microstrip line can be used as a simple signal transmission line. In addition, if a suitable patterning is applied, the microstrip line can be used as microwave components including an inductor, a capacitor, a filter, a resonator, a delay line and a transistor etc. Accordingly, improvement of the microstrip line will lead to improvement of characteristics of the microwave component.
In addition, the oxide superconductor material (high T.sub.c copper oxide superconductor) which has been recently discovered makes it possible to realize a superconducting state at temperatures achievable by low cost liquid nitrogen cooling. Therefore, various microwave components using an oxide superconductor have been proposed.
It is well known that lumped components are favorable in their size compared with distributed components. Due to their small size, the lumped components can be easily combined with other distributed or lumped components so as to form hybrid circuits.
By using the oxide superconductors for the lumped components, the dissipation and dispersion is considered to be significantly smaller than those of conventional metals or semiconductors.
However, it is almost impossible to change properties of the lumped components after they are assembled into circuits.